1. Field of the Invention
This invention relates to substantially colorless, organic polyisocyanates containing about 0.01 to 1 ppm (by weight) of an organic dye (mixture) soluble in the polyisocyanate and having an absorption maximum of about 560 to 590 nm, to processes for producing these colorless organic polyisocyanates and to their use in the production of polyisocyanate polyaddition products, preferably polyurethane plastics, by the isocyanate polyaddition process.
2. Description of the Prior Art
Organic polyisocyanates, particularly those containing aliphatically and/or cycloaliphatically bound isocyanate groups, are widely used in practice, particularly in the production of light-stable polyurethane plastics and lacquer coatings characterized by extremely high fastness to light, high chalking resistance and excellent gloss retention. In practice, modification products (of simple aliphatic or cycloaliphatic diisocyanates) having a reduced vapor pressure are generally used in those fields. Modification products of the type in question are, for example, polyisocyanates of reduced vapor pressure containing allophanate, uretdione, urea, semicarbazide, urethane and, in particular, biuret or isocyanurate structural units. In the commercial production of these modification products from the corresponding simple diisocyanates, the inadequate color stability under heat of the reaction mixtures often leads to discolored, generally yellowish to yellow polyisocyanates. This yellowing, particularly in the field of lacquers, frequently prevents uniform coloring, especially in the case of pigmented metallic lacquers.
It has now been found that these disadvantages can be obviated by incorporating in the polyisocyanates very small quantities of the dyes described in detail hereinafter which are soluble therein so that substantially colorless polyisocyanates are obtained from the yellowish to yellow polyisocyanates.